The present invention relates to the direct sulphidization fuming of zinc and more particularly to the treatment of zinc bearing feed materials.
Zinc sulphide such as sphalerite mineral, is often the starting point for production of zinc metal or zinc oxide and conventionally it is roasted in an oxidizing environment to produce zinc oxide ZnO. The production of high grade zinc oxide from roasting is not usually possible if copper sulphide (Cu.sub.2 S) or iron sulphide (FeS or FeS.sub.2) are present in the mineralization. In the case of iron the formation of zinc ferrite complicates the subsequent extraction by leaching of a high grade zinc oxide.
Known alternative ways of recovering zinc from sulphide minerals such as sphalerite involve the use of pressure leaching operations such as the Sherritt-Cominco process. However, these are problematic again in the presence of iron due to the formation of jarosites in the effluent. The presence of copper at any significant level also complicates subsequent extraction of zinc.
Another pyrometallurgical method of recovering zinc oxide is by fuming from slags, often derived from lead smelting operations. In this case, the zinc oxide is recovered by reducing the zinc in the slag at high temperature to zinc vapour by the addition of carbon, carbon monoxide, methane or other reducing agent to the molten slag. This process is very energy intensive and can only be applied where the zinc is present in slag as metal or oxide. An example of this type of prior art process is described in Australian Patent Applications 61527/86 (Pyrotech Resources N.L. and Ausmelt Pty Ltd).
Yet another process for generating zinc oxide pyrometallurgically makes use of the Waelz kiln in which the zinc-bearing feedstock, is passed with coke through a high temperature horizontal kiln in which reduction of the zinc oxide to zinc vapour occurs. The zinc vapour leaves the kiln and is subsequently oxidized downstream to a zinc oxide product. This process cannot produce a high quality zinc oxide due to the physical entrainment and carryover of other material with the fumed zinc.
Yet another proposed prior art process involves the treatment of lead-zinc complex sulphide ores or concentrates by a reductive fuming process. The process as proposed in Australian Patent Application AU-B-79186/87 (CSIRO) involves adding the feed to a bath of molten matte which is overlaid by a slag layer; agitating and heating the bath; and adding a reductant to the bath. The addition of a reductant to the bath is unnecessary and undesirable from the point of view of excess reagent use and handling excessive heat loads in downstream equipment. In addition the recovery rates of the CSIRO process are relatively low and operating costs high and it is not clear whether the process could be applied in the absence of significant amounts of lead.
It is well known that tin can be recovered efficiently from slags add low-grade tin concentrates by fuming using sulphidization in which sulphur is added in the form of pyrite (FeS2) to the feed material. This process is based on the extreme volatility of tin as its sulphide. It is also known that sphalerite (zinc sulphide) may be used as a sulphidizer to fume tin. However, this is not often a desirable fuming agent since tin and zinc are difficult to separate and a proportion of the zinc is found in the end product.